I. Field of the Invention
The present invention relates generally to mobile radio communication systems. More specifically, the present invention relates to code division multiple access (CDMA) mobile radio communication systems that control the output transmit power of information signals sent between mobile units and base stations.
II. Description of the Related Art
In CDMA radio communication systems operating in accordance with the TIA/EIA-95 standard (the IS-95 standard), the base station transmits a power control stream at 800 bps to the mobile station on the Traffic Channel. Binary phase shift keying (BPSK) modulation is used for the power control bit stream. One phase of the modulated power control signal indicates that the mobile station is to increase transmit power. Another phase of the modulated power control signal indicates that the mobile station is to decrease transmit power. The base station determines whether to have the mobile station increase or decrease its transmit power by comparing a measured value (Z) that is related to the actual received signal to noise ratio (E.sub.c /N.sub.t).sub.actual at the base station to a threshold which corresponds to a desired signal to noise ratio (E.sub.c /N.sub.t).sub.desired at the base station. If the value (Z) is less than the threshold, then the base station sends a command to the mobile station instructing it to increase its transmit power. If this value (Z) is greater than the threshold, then the base station sends a command to the mobile station instructing it to decrease its transmit power. This power control system is described more fully in IS-95 and in U.S. Pat. Nos. 5,056,109 and 5,265,119, both of which are entitled "Method and Apparatus for Controlling Transmission Power In A CDMA Cellular Telephone System" and assigned to the assignee of the present invention and incorporated by reference herein. Further details of an exemplary CDMA system can be found in U.S. Pat. No. 4,901,307 entitled "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters," which is assigned to the assignee of the present invention and incorporated herein in its entirety by reference. The IS-95 standard is set forth in the TIA/EIA Interim Standard entitled "Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,"TIA/EIA/IS-95, dated July, 1993, the contents of which are also incorporated herein by reference.
As mentioned above, in CDMA systems that comply with the IS-95 standard, the base station compares the measured value (Z) which is related to the actual received signal to noise ratio (E.sub.c /N.sub.t).sub.actual at the base station to the threshold (E.sub.c /N.sub.t).sub.desired in order to determine whether to send a power up or a power down command to the mobile station. Although the value of Z is related to the actual received signal to noise ratio (E.sub.c /N.sub.t).sub.actual, Z is in fact randomly distributed around the value (E.sub.c /N.sub.t).sub.actual as a result of the random noise component in the received signal at the base station. As a result of this, the measured value of Z at any given time will likely be different from the actual received signal to noise ratio (E.sub.c /N.sub.t).sub.actual. An example of this is shown in FIG. 1, where curve "a" shows the random distribution of Z around the (E.sub.c /N.sub.t).sub.actual value. (The distribution is assumed to be Guassian for purposes of discussion). Unfortunately, as shown in FIG. 1, when the actual signal to noise ratio (E.sub.c /N.sub.t).sub.actual is sufficiently close to the desired signal to noise ratio (E.sub.c /N.sub.t).sub.desired threshold, a portion of the distribution (identified as region "b" in FIG. 1) for Z will lie on the opposite side of the threshold from the (E.sub.c /N.sub.t).sub.actual value. Significantly, when the Z lies in this region "b" on FIG. 1, the base station will send an erroneous power down command to the mobile station. The power down command will be erroneous, because the actual received signal to noise ratio is on the side of the threshold (E.sub.c /N.sub.t).sub.desired associated with a power up command, rather than a power down command. In view of the example shown in FIG. 1, it is clear that, as a general rule, the likelihood that the power control command from the base station will represent an erroneous value will increase as the actual received signal to noise ratio (E.sub.c /N.sub.t).sub.actual approaches the desired signal to noise ratio (E.sub.c /N.sub.t).sub.desired. The transmission of such erroneous power control commands is undesirable because misleading information is being sent to the mobile station (e.g., the mobile station is being commanded to power down when it should be increasing its power). In addition, given that such power control commands are erroneous, the allocation of transmit power on the forward link to support transmission of these commands represents a waste of forward link transmit power which is limited in CDMA systems. It would therefore be desirable if the transmission of such erroneous power control information could be minimized or eliminated.
Another aspect of the reverse link power control system used in IS95 and described above results from the fact that a power control bit (indicating whether the mobile station should either increase or decrease its transmit power) is transmitted to each and every mobile station. It would be desirable, especially in cases where the base station determined that the actual received signal to noise ratio (E.sub.c /N.sub.t).sub.actual from a particular mobile station was within a range of the desired signal to noise ratio (E.sub.c /N.sub.t).sub.desired such that there was a significant probability that an erroneous power control command would be sent from the base station, for the base station to be able to reduce the power used for transmitting the power control information to the particular mobile station, thereby conserving transmit power at the base station and either allowing more transmit power to be allocated to information that is being transmitted to other mobile stations or allowing more users to be supported by the base station.
A still further aspect of the reverse link power control system used in IS95 that is relevant to an understanding of the disclosed method and apparatus is the performance of the mobile station when the mobile station receives a weak (e.g., low signal to noise ratio) power control command from a base station. In such cases, due to the weakness of the signal, the mobile station will have difficulty determining whether the base station sent a power up or alternatively a power down command to the mobile station. In these cases, the mobile station will attempt to decode the power control command, but will often arrive at an erroneous result due to the weakness of the signal. In other words, after attempting to decode the weak power control command, the mobile station will for example decide that the base station had sent a power up command to the mobile station when in fact the base station had sent a power down command to the mobile station. As a general rule, it is believed that the weaker the power control command from the base station, the more likely it is that the mobile station will erroneously interpret the power control command. Moreover, it is believed that, when the power control command sent from the base station becomes sufficiently weak, the error rate of the mobile station in determining whether the power control command from the base station was a power up command or a power down command, may approach 50%.
These problems and deficiencies in the reverse link power control system of IS-95 are recognized and overcome by the improved system for conserving transmit power disclosed below.